ROCKS AND MINERALS — FORMATION, TYPES, PARENT MATERIAL AND PROCESSES
This is for Agriculture Optional, M.Sc Entrance, AFO, IFFCO, JRF, NABARD, RRB-SO, and STATE Exam.
Rocks are the materials that form the essential part of the Earth’s solid crust.
“Rocks are hard masses of mineral matter comprising one or more rock-forming minerals”.
Rocks are formed from the molten material known as magma. The study of rocks is called Petrology.
Petrology deals with the description of rocks; petrogenesis is the study of the origin of rocks.
Minerals that are original components of rocks are called primary minerals. (Feldspar, mica, etc.).
Minerals formed from changes in primary minerals and rocks are called secondary minerals (clay minerals).
Those minerals that are chief constituents of rocks are called essential minerals (Feldspars, pyroxenes micas etc)
Those present in small quantities, whose presence or absence will not alter the properties of rocks are called accessory minerals (tourmaline, magnetite etc).
Formation of Rocks
- Cooling and consolidation of molten magma within or on the surface of the earth =Igneous or Primary Rocks
- Transportation and cementation of primary rocks = Sedimentary or Secondary Rocks
- Alteration of the existing primary and secondary rocks = Metamorphic Rocks
Types of Parent Material
Parent Material develops from plant residue knowns as Cumulose. Ex- Peat and Muck.
- Colluvial: By Gravity
- Alluvial, Marine: By Water
- Aeolian: By Wind (Sand)
- Glacial: By Ice
- Eoline: By Wind (Silt)
- Lacustrine: Sedimentation of minerals in free water like a lake.
Best soil contains 10–20% clay, 5–10% OM, 30% Silt or Sand called clay Loam.
A soil is considered to be fertile when the base saturation percentage is more than 80. Each per cent of humus contributes about 2 C mol /kg of CEC.
The decomposition of organic matter is accelerated in warm climates as compared to cooler climates. For each 10-degree C decline in mean annual temperature, the total organic matter and N increase by two to three times.
Naturally occurring aggregates of soil are called — Peds.
Artificially formed (by Tillage) called- Clod.
Soil Factors and Processes
Soil formation is the process of two consecutive stages.
The first step is accomplished by weathering (disintegration & decomposition)
— Rock to Regolith to True Soil (Weathering of Rocks)
The second step is associated with the action of Soil Forming Factors.
— Soil factors divide into two groups
- Passive: Parent material, Relief and Time
- Active: Climate and Vegetation & organism
Soil Forming Processes: Humification, Eluviation (O & A Horizon), Illuviation ( B horizon), Horizonation, Calcification, Decalcification, Podzolization, Lateralization, Gleization, Salinization.
- Humification: Humification is the process of transformation of raw organic matter into humus. It is an extremely complex process involving various organisms.
- Eluviation: Eluviation means washing out. It is the process of removal of constituents in suspension or solution by the percolating water from the upper to lower layers. (O & A Horizon)
- Illuviation: This means washing in. The process of deposition of soil materials in the lower layer is termed Illuviation. (B horizon)
- Horizonation: It is the process of differentiation of soil in different horizons along the depth of the soil body.
- Calcification: It is the process of precipitation and accumulation of calcium carbonate (CaCO3) in some parts of the profile.
- Decalcification: It is the reverse of calcification which is the process of removal of CaCO3 or calcium ions from the soil by leaching.
- Podzolization: Accumulation of Silica and leaching of sesquioxide (Al & Fe ) into the lower layers.
- Lateralization: Lateralization is the process that removes silica, instead of sesquioxide from the upper layers and thereby leaving sesquioxide to concentrate in the solum. (Removal of Silica and Accumulation of Sesquioxide ( Al & Fe )
- Gleization: Gleization is a process of soil formation resulting in the development of a glei (or gley horizon) in the lower part of the soil profile above the parent material due to poor drainage conditions (lack of oxygen) and where waterlogged conditions prevail.
- Salinization: It is the process of accumulation of salts, such as sulphates and chlorides of calcium, magnesium, sodium and potassium, in soils in the form of a salty (salic) horizon. It is quite common in arid and semi-arid regions.
- Desalinization: It is the removal by leaching of excess soluble salts from horizons or soil profiles by ponding water and improving the drainage conditions by installing an artificial drainage network.
Other Terms
Dealkalization (Solodization): The process refers to the removal of Na+ from the exchange sites. This process involves the dispersion of clay. Dispersion occurs when Na+ ions become hydrated, the process is affected by intensive leaching and degradation which takes place in older soils.
Solonization or Alkalization: The process involves the accumulation of sodium ions on the exchange complex of the clay, resulting in the formation of sodic soils.
Pedoturbation: It is the process of mixing the soil.
